During the operation of high temperature reaction vessels in the steel industry, material treatment devices are introduced through a neck in the vessel. As these devices deliver gaseous and/or solid materials to the vessel there are sudden bursts of high temperature gases. In order to contain fumes and maintain a desired atmosphere within the vessel, it is desirable to form a seal between the material treatment device and the vessel. This seal should optimally allow for movement of the material treatment device including its removal for maintenance while preventing volatile gases from escaping. Common methods of sealing a lance in a reaction vessel are often problematic because the seal is only effective when the axis of the sealing mechanism coincides precisely with the axis of the mouth of the vessel. U.S. Pat. No. 4,465,265 discloses an adjustably positioned lance structure which provides a gas-tight passage comprising a metal bellows and a protective sleeve which projects into the reactor. U.S. Pat. No. 5,377,960 describes an oxygen/carbon blowing lance assembly which includes a modular design housing for ease of maintenance but it is not disclosed how the lance is sealed in the vessel.